Geotechnical Engineering Office, Civil Engineering and Development Department
The Government of the Hong Kong Special Administrative Region
GEO Technical Guidance Note No. 43 (TGN 43)
Guidelines on Hydraulic Design of U-shaped and Half-
Slopes
1. SCOPE
1.1 This Technical Guidance Note (TGN) provides technical guidance on the hydraulic
design of U-shaped and half-round channels on slopes. It supersedes the design
methodology for U-shaped and half-round channels given in the second paragraph of
Section 8.3.4 and the chart given in Figure 8.7 of the Geotechnical Manual for Slopes
(GCO, 1984).
1.2 Any feedback on this TGN should be directed to Chief Geotechnical Engineer/Standards
& Testing of the GEO.
2. TECHNICAL POLICY
2.1 The technical recommendations promulgated in this TGN were agreed by GEO
Geotechnical Control Conference on 30 May 2014.
3. RELATED DOCUMENTS
3.1 Ackers, P. (1969). Charts for the Hydraulic Design of Channels and Pipes. Hydraulic
Research Station, Wallingford, Hydraulics Research Paper No. 2, 46 p.
3.2 DSD (2013). Stormwater Drainage Manual (with Eurocodes incorporated) - Planning,
Design and Management (Fourth Edition). Drainage Services Department, Hong Kong,
172 p.
3.3 GCO (1984). Geotechnical Manual for Slopes (Second Edition). Geotechnical
Engineering Office, Hong Kong, 302 p. (Reprinted, 2011)
3.4 HR Wallingford (2006). Tables for the Hydraulic Design of Pipes, Sewers and Channels
(8th Edition). HR Wallingford, DIH Barr, 2006, Thomas Telford.
4. BACKGROUND
4.1 Second paragraph of Section 8.3.4 of GCO (1984) recommends the use of Manning’s
formula, with a maximum permissible velocity of 4 m/s and a roughness factor of 0.013,
for design of U-shaped and half-round channels on slopes and provides a chart in
Figure 8.7 for the rapid design of channels. It further recommends that channels larger
than 600 mm may be designed using the charts developed by Ackers (1969) based on the
Colebrook-White equation.
4.2 A review of the design methodology for both U-shaped and half-round channels given in
Section 8.3.4 and the chart given in Figure 8.7 of GCO (1984) has been carried out. It is
found that the use of Manning’s formula and Colebrook-White equation for design of